Metal borides are chemically stable and exhibit various electric properties according to the content of boron. Thus, it is expected that a variety of applications would be developed. Among them, especially lanthanum hexaboride (LaB6) has low work function. Accordingly, the application has further been developed as an electrode material in an electron emitter, a light, and the like. The work function indicates minimum energy necessary for taking out electrons from the surface of material. The lower work function of an electron emitter is more preferable.
The LaB6 is often used as a film. Various methods of producing the LaB6 film have been examined. Among these methods, the sputtering method of producing a dense film by using an LaB6 target is suitably used. The LaB6 film should have high purity and high crystallinity in order to have a lower work function.
Generally, the target should have high density and high purity. A conventional LaB6 target is produced by sintering a commercially available LaB6 powder.
However, the commercially available LaB6 powder contains lanthanum oxide, boron oxide, and lanthanum-boron complex oxide as impurities in 1.5 mass % or more in oxygen equivalent and lanthanum carbide and boron carbide in 0.2 mass % or more in carbon equivalent.
Lanthanum oxide and boron oxide contained in a commercially available LaB6 powder is LaB6 oxidized mainly with oxygen of the atmosphere. The industrial manufacturing process of a commercially available LaB6 powder requires the process of pulverizing synthesized powder to have a particle-size suitable as the raw material of a sintered body, so that it is realistically impossible to handle LaB6 powder completely free from the atmosphere. The fracture surface newly generated by this pulverization reacts with oxygen of the atmosphere so as to be lanthanum oxide or boron oxide.
Lanthanum boron complex oxide contained to a commercially available LaB6 powder is generated by reacting lanthanum oxide contained in various La raw materials with boron oxide contained in B raw material during the synthesis of a commercially available LaB6 powder.
On the other hand, lanthanum carbide and boron carbide are generated due to carbon added during the synthesis. The carbon is added to reduce various La raw materials into La metals reactable with B during the synthesis. Since excessive carbon is usually added to react with La completely, the remaining carbon is reacted with La and B to be lanthanum carbide and boron carbide respectively. It is realistically impossible to avoid the generation of these carbides. Therefore, it is very difficult to avoid lanthanum oxide, boron oxide, lanthanum boron complex oxide, lanthanum carbide, boron carbide, and residual carbon from being contained in a commercially available LaB6 powder manufactured by an industrial production method.
Impurities contained in these LaB6 powder cannot be removed even by being heated at a sintering temperature, remaining in the grain boundary of an LaB6 sintered body. Accordingly, a conventional LaB6 sintered body produced by using a commercially available LaB6 powder contains impurities derived from the raw powder in at least 3 volume %. Impurities contained to the target are directly incorporated in a sputtering film. Since the impurities have a higher work function than LaB6, the work function of an LaB6 film containing such impurities is increased.
Therefore, there has been the problem in which the use of a conventional LaB6 target produced by sintering a commercially available LaB6 powder increases the impurity content of the obtained LaB6 film resulting in a higher work function.
To decrease the impurity content of the target, the method of sintering LaB6 powder with high purity is effective. The impurities contained in LaB6 powder can be effectively removed by being hot-cleaned with an inorganic acid. The use of a target highly purified by this acid cleaning can form an LaB6 film with a small amount of impurities.
On the other hand, an LaB6 film has been examined for application to an electron emitter, an electrode material, and the like. For this LaB6 film, a metal substrate, a glass substrate, an Si wafer, and the like are mainly used for a film-formed substrate. There has been the problem in which when a highly purified LaB6 sintered body is used for a target to form a film on the substrate, the obtained film has high purity but poor crystallinity and high work function and thus is peeled easily.
Generally, when a film is formed by sputtering on a film-formed substrate with a composition and a crystal structure which are different from those of a film respectively, the internal stress is generated in a sputtering film due to the difference in the physical properties of between the substrate and the film, resulting in the poor crystallinity of the film.
This phenomenon occurs even on an LaB6 sputtering film, depending on the material of a substrate. This tendency is significant when the coefficient of thermal expansion and the lattice constant are substantially different from those of LaB6. The crystallinity of an LaB6 film deteriorates as the internal stress of the film is increased. It is known that the crystallinity of a film is related to the work function. Thus, the decrease of the crystallinity leads to the increase of the work function. Furthermore, a film is peeled easily from the substrate as the internal stress of a film is increased, so that it is impossible to form the film.
When an LaB6 sputtering film contains impurities capable of alleviating the respective differences in the coefficient of thermal expansion and the lattice constant of between LaB6 and a substrate, the internal stress of the film can be decreased.
However, such impurities have a higher work function than LaB6. Thus, impurities contained an LaB6 film relax the internal stress of the film but cannot avoid the work function from being increased.
Therefore, a target material capable of producing an LaB6 sputtering film with high adhesion and high crystallinity has been desired, which does not contain impurities increasing the work function but alleviates the difference in the thermal expansion between LaB6 and a substrate.
Furthermore, due to the sintering resistance of LaB6, a conventional LaB6 target formed by sintering a commercially available powder has only a relative density of about 80% and a large number of pores. An organic component, water, and the like are typically adsorbed in these pores. Then, a problem arises, in which during sputtering, this organic component and water are discharged in a vacuum chamber to contaminate the chamber and then are incorporated in a sputtering film to cause degradation of the film. To curb the influence of these pores, the LaB6 spatter target has preferably a relative density of 88% or more.
There is a method of densifying a sintered body by sintering lanthanum hexaboride powder to which a sintering additive is added (for example, see PLTs 1 and 2). In this case, since metal oxide is typically used as a sintering additive, the metal oxide remains in the sintering body as impurities after sintering. When this sintered body is used as a target material, the impurities in the sintering body are incorporated in a sputtering film to cause degradation of the film.
On the other hand, a target for sputtering using metal boride containing one kind or more selected from hafnium boride, titanium boride, tungsten boride, and lanthanum hexaboride in major proportions and a method of producing the same is disclosed, in which the density ratio of the sintering body is 80% or more and in which the crystal size is 50 μm or less (for example, see PLT 3).
This technology obtains a high density boride target by significantly decreasing the interparticle spaces to improve the relative density. This technology is to improve the mass production of the products manufactured by using this target but does not mention the high purification of the target.